Problem A. 558. (March 2012)
A. 558. Prove that there exists a constant C>0 for which the following statement holds: if n is a positive integer and are sets such that every two of them has at least two, and every three of them has at most three elements in common, then N<Cn^{2}.
(Proposed by: Z. Gyenes, Budapest)
(5 pont)
Deadline expired on 10 April 2012.
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